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Diamagnetic Persistent Current in Diffusive Normal-Metal Rings E University of South Carolina Scholar Commons Faculty Publications Physics and Astronomy, Department of 2-19-2001 Diamagnetic Persistent Current in Diffusive Normal-Metal Rings E. M.Q. Jariwala P. Mohanty M. B. Ketchen Richard A. Webb University of South Carolina - Columbia, [email protected] Follow this and additional works at: https://scholarcommons.sc.edu/phys_facpub Part of the Physics Commons Publication Info Published in Physical Review Letters, Volume 86, Issue 8, 2001, pages 1594-1597. Jariwala, E.M.Q., Mohanty, P., Ketchen, M.B., and Webb, R.A. (2001). Diamagnetic Persistent Current in Diffusive Normal-Metal Rings. Physical Review Letters, 86(8), 1594-1597. doi: 10.1103/PhysRevLett.86.1594 © 2001 The American Physical Society. This Article is brought to you by the Physics and Astronomy, Department of at Scholar Commons. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. VOLUME 86, NUMBER 8 PHYSICAL REVIEW LETTERS 19FEBRUARY 2001 Diamagnetic Persistent Current in Diffusive Normal-Metal Rings E. M. Q. Jariwala,1 P. Mohanty,1,* M. B. Ketchen,2 and R. A. Webb1 1Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, Maryland 20742 2IBM T. J. Watson Research Center, Yorktown Heights, New York 10598 (Received 9 December 1999) We have measured a diamagnetic persistent current with flux periodicities of both h͞e and h͞2e in an array of thirty diffusive mesoscopic gold rings. At the lowest temperatures, the magnitudes of the currents per ring corresponding to the h͞e- and h͞2e-periodic responses are both comparable to the Thouless energy Ec ϵ h¯͞tD, where tD is the diffusion time. Taken in conjunction with earlier experiments, our results strongly challenge the conventional theories of persistent current. We consider a new approach associated with the saturation of the phase coherence time tw. DOI: 10.1103/PhysRevLett.86.1594 PACS numbers: 73.23.Ra, 05.40.–a, 73.50.Mx p ͞ The phase of the electron wave function lies at the to e2 T T0 , and compare the different characteristic tem- heart of many mesoscopic phenomena. Consider a phase- peratures T0 to Ec. The sum of all the experimental results coherent ring of length L: the presence of an Aharonov- prompts a discussion of new calculations based on a direct Bohm flux F introduces a phase factor into the boundary connection to the saturation of tw at low temperatures. ͞ condition C͑x 1 L͒ ෇ C͑x͒e2piF F0 , such that all ther- Figure 1(a) displays a section of our array of 30 Au modynamic quantities are oscillatory in the applied flux, rings, fabricated using e-beam lithography on top of one with a period F0 ෇ h͞e. To minimize its free energy F, arm of the counterwound Nb pickup coil of a dc SQUID. this isolated ring will support an equilibrium persistent cur- A 150-nm-thick layer of SiO separates the coils from the rent I ෇ 2≠F͞≠F, even in the presence of disorder [1]. Au rings. We compute that the mutual inductance of each Moreover, for an ensemble of such rings, the fundamen- ring to the coil is 2.7 pH and uniform to within 5%. The tal harmonic of this current (with flux periodicity h͞e) gradiometer configuration of the two counterwound coils is strongly suppressed, whereas the next harmonic (with is clearly displayed in Fig. 1(b) together with the pair of flux periodicity h͞2e) survives due to the contribution of on-chip Nb field coils. Operating the dc SQUID in a flux- time-reversed paths of the electron [2]. locked loop mode, we measure a typical flux noise of Nanoampere-sized h͞e-orh͞2e-periodic currents have been separately observed in a few experiments: the h͞e current has been measured in single Au rings [3] and in a GaAs-AlGaAs ring [4], while the h͞2e current has been measured in multiring arrays of 107 Cu rings [5] and 105 GaAs-AlGaAs rings [6], where the h͞e response was not observable. Except for the case of the nearly ballistic GaAs ring [4], all the currents measured are remarkably 10–100 times larger than theoretically expected [7–9]. The dia- magnetic sign of the average h͞2e response at high fre- quencies in Ref. [6] also contrasts with most predictions. Disorder plays a strong role in these results, emphasizing the Thouless scale of electron diffusion Ec. In this Letter, we report the first experiment on a single sample to measure the sign, amplitude, and temperature dependence for both the h͞e and h͞2e components of the persistent current. For our array of 30 diffusive Au rings, we determine the sign of both currents to be dia- magnetic near zero field. At our lowest temperature of 5.5 mK, the amplitudes of the total h͞e and h͞2e cur- rents are ͗I1T ͘ ෇ 1.92 nA and ͗I2T ͘ ෇ 1.98 nA, respec- tively. For comparison to previous experiments, wep present our results for the currents per ring as ͗I1͘ ෇ ͗I1T ͘͞ 30 ෇ ෇ ͞ ͗ ͘ ෇ ͗ ͘͞ ෇ ෇ 0.35 nA 2.3eEc h¯ and I2 I2T 30 0.066 nA FIG. 1. (a) Electron micrograph showing the Au rings, as well 0.44eEc͞h¯. We fit the temperature dependence of both the as the pickup and field coils of the SQUID. (b) Larger view ͞ h͞e and h͞2e currents to the form e2T T0 [8], as well as illustrating the entire gradiometer design of the SQUID. 1594 0031-9007͞01͞86(8)͞1594(4)$15.00 © 2001 The American Physical Society VOLUME 86, NUMBER 8 PHYSICAL REVIEW LETTERS 19FEBRUARY 2001 p ϳ3mFS͞ Hz down to 2 Hz at low temperatures. Here FS denotes the superconducting flux quantum. Each ring has a perimeter of length L ෇ 8.0 6 0.2 mm, with a line thickness t ෇ 60 6 2 nm and linewidth w ෇ 120 6 20 nm. We estimate Rᮀ ෇ 0.15 V͞ᮀ and a 2 diffusion constant of D ෇ yFle͞2 ෇ 0.06 m ͞s, where the elastic mean free path le Ӎ 87 nm. These parametersp as well as the phase coherence length Lw ϵ Dtw are measured separately in a 207 mm long meander with the same line thickness and line width as the rings. By using a standard weak localization measurement of the magnetore- sistance, we determined that Lw ഠ 16 mm for this control sample and is temperature independent below 500 mK 2 [10]. The Thouless energy Ec ϵ h¯͞tD ෇ hD¯ ͞L is 7.3 mK; the mean energy level spacing is given by D ϵ 1͞2N0V ෇ 19 mK, where N0 is the density of states and V is the volume of each ring. We measure the response of the rings to an applied magnetic field as a change in the average magnetic mo- ment coupled to the SQUID pickup coil. To distinguish the flux-periodic current in the rings from the background magnetization, we modulate the field at a small frequency f and measure separately the first, second, and third har- monics at f, 2f, and 3f, respectively. The amplitudes of the periodic signals measured are related to the ac modu- lation amplitude by a Bessel function, allowing us to maximize the signal in a particular harmonic for either ͞ ͞ FIG. 2. Magnetic response of 30 Au rings from a run focusing the h e or the h 2e flux-periodic response [11]. The on h͞2e oscillations. The dashed line in (a) represents raw response is independent of the measurement frequency f data. The solid line represents the h͞2e-periodic contribution, below 12 Hz, as also seen previously [3]. taken from the h͞2e window in the power spectrum in (b). In the following, we discuss a typical data set, shown The autocorrelation shown in (c) also demonstrates the h͞2e- in Fig. 2, which focuses on the h͞2e oscillations in the periodic signal. second harmonic of the magnetization using a 2.26 G ac modulation. As in most of our runs, we apply this ac field tion is consistent withp the corresponding weak-localization at f ෇ 2.02 Hz, which is superimposed on a dc field. The field scale of Hc ෇ 3¯h͞ewL. dc field is swept slowly over 620 G at 3.6 G͞h. The data A similar decaying envelope is seen in the h͞e-periodic is averaged in 0.08 G bins for over 80 s per bin, resulting response, as shown in Fig. 3(a). Here we use a 5.38 G ac in a noise floor of ϳ0.3mFS for each bin. In Fig. 2(a) modulation to maximize the h͞e oscillations in the third we display the raw data (dashes) of the second harmonic harmonic at 6.06 Hz. The solid line in Fig. 3(a) represents component at frequency 4.04 Hz taken at the base tem- the h͞e-periodic contribution to the total signal. Note that perature of 5.5 mK, along with the h͞2e-periodic contri- the leftmost peak shown in the power spectrum in both bution (solid line) represented by the signal in the h͞2e Figs. 2(b) and 3(b) results from the finite field range of window of the power spectrum shown in Fig. 2(b). The our data with a background subtracted. The size of our width of these windows corresponding to h͞e-orh͞2e- rings has been chosen to avoid confusion of the h͞e-or periodic currents reflect the finite linewidth of the rings; h͞2e-periodic signals with this lower-frequency response. furthermore, they are broadened to account for the effect The data in Figs. 2(a) and 3(a) also illustrate the dia- of the intrinsic width of the transform due to the finite field magnetic sign of the response near zero field.
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